Rapamycin (sirolimus) (FIG. 1) is a lipophilic macrolide produced by Streptomyces hygroscopicus NRRL 5491 (Sehgal et al. 1975; Vézina et al., 1975; U.S. Pat. Nos. 3,929,992; 3,993,749) with a 1,2,3-tricarbonyl moiety linked to a pipecolic acid lactone (Paiva et al., 1991). For the purpose of this invention rapamycin is described by the numbering convention of McAlpine et al. (1991) in preference to the numbering conventions of Findlay et al. (1980) or Chemical Abstracts (11th Cumulative Index, 1982-1986 p60719CS).
Rapamycin has significant pharmacological value due to the wide spectrum of activities exhibited by the compound. Rapamycin shows moderate antifungal activity, mainly against Candida species but also against filamentous fungi (Baker et al., 1978; Sehgal et al., 1975; Vézina et al., 1975; U.S. Pat. Nos. 3,929,992; 3,993,749). Rapamycin inhibits cell proliferation by targeting signal transduction pathways in a variety of cell types, e.g. by inhibiting signalling pathways that allow progression from the G1 to the S-phase of the cell cycle (Kuo et al., 1992). In T cells rapamycin inhibits signalling from the IL-2 receptor and subsequent autoproliferation of the T cells resulting in immunosuppression. The inhibitory effects of rapamycin are not limited to T cells, since rapamycin inhibits the proliferation of many mammalian cell types (Brunn et al., 1996). Rapamycin is, therefore, a potent immunosuppressant with established or predicted therapeutic applications in the prevention of organ allograft rejection and in the treatment of autoimmune diseases (Kahan et al., 1991). 40-O-(2-hydroxy)ethyl-rapamycin (SDZ RAD, RAD 001, Certican, everolimus) is a semi-synthetic analogue of rapamycin that shows immunosuppressive pharmacological effects and is also under investigation as an anticancer agent (Sedrani, R. et al., 1998; Kirchner et al., 2000; U.S. Pat. No. 5,665,772, Boulay et al, 2004). Approval for this drug as an immunosuppressant was obtained for Europe in 2003. The rapamycin ester derivative CCI-779 (Wyeth-Ayerst) inhibits cell growth in vitro and inhibits tumour growth in vivo (Yu et al., 2001). CCI-779 is currently in Phase III clinical trials as a potential anti-cancer agent. The value of rapamycin in the treatment of chronic plaque psoriasis (Kirby and Griffiths, 2001), the potential use of effects such as the stimulation of neurite outgrowth in PC12 cells (Lyons et al., 1994), the block of the proliferative responses to cytokines by vascular and smooth muscle cells after mechanical injury (Gregory et al., 1993) and its role in prevention of allograft fibrosis (Waller and Nicholson, 2001) are areas of intense research (Kahan and Camardo, 2001). Recent reports reveal that rapamycin is associated with a lower incidence of cancer in organ allograft patients on long-term immunosuppressive therapy than those on other immunosuppressive regimes, and that this reduced cancer incidence is due to inhibition of angiogenesis (Guba et al., 2002). It has been reported that the neurotrophic activities of immunophilin ligands are independent of their immunosuppressive activity (Steiner et al., 1997) and that nerve growth stimulation is promoted by disruption of the mature steroid receptor complex as outlined in the patent application WO 01/03692. Side effects such as hyperlipidemia and thrombocytopenia as well as potential teratogenic effects have been reported (Hentges et al., 2001; Kahan and Camardo, 2001).
The polyketide backbone of rapamycin is synthesised by head-to-tail condensation of a total of seven propionate and seven acetate units to a shikimate derived cyclohexanecarboxylic acid starter unit by the very large, multifunctional proteins that comprise the Type I polyketide synthase (rap PKS, Paiva et al., 1991). The L-lysine derived amino acid, pipecolic acid, is condensed via an amide linkage onto the last acetate of the polyketide backbone (Paiva et al., 1993) and is followed by lactonisation to form the macrocycle.
The nucleotide sequences for each of the three rapamycin PKS genes, the NRPS-encoding gene and the flanking late gene sequences and the corresponding polypeptides, were identified by Aparicio et al., 1996, and Schwecke et al., 1995 and were deposited with the NCBI under accession number X86780, and corrections to this sequence have recently been published in WO 04/007709.
The first enzyme-free product of the rapamycin biosynthetic cluster has been designated pre-rapamycin (WO 04/007709, Gregory et al., 2004). Production of the fully processed rapamycin requires additional processing of the polyketide/NRPS core by the enzymes encoded by the rapamycin late genes, RapJ, RapN, RapO, RapM, RapQ and RapI.
The pharmacologic actions of rapamycin characterised to date are believed to be mediated by the interaction with cytosolic receptors termed FKBPs. The major intracellular rapamycin receptor in eukaryotic T-cells is FKBP12 (DiLella and Craig, 1991) and the resulting complex interacts specifically with target proteins to inhibit the signal transduction cascade of the cell.
The target of the rapamycin-FKBP12 complex has been identified in yeast as TOR (target of rapamycin) (Alarcon et al., 1999) and the mammalian protein is known as FRAP (FKBP-rapamycin associated protein) or mTOR (mammalian target of rapamycin) (Brown et al. 1994).
A link between mTOR signalling and localized protein synthesis in neurons; its effect on the phosphorylation state of proteins involved in translational control; the abundance of components of the translation machinery at the transcriptional and translational levels; control of amino acid permease activity and the coordination of the transcription of many enzymes involved in metabolic pathways have been described (Raught et al., 2001). Rapamycin sensitive signalling pathways also appear to play an important role in embryonic brain development, learning and memory formation (Tang et al., 2002). Research on TOR proteins in yeast also revealed their roles in modulating nutrient-sensitive signalling pathways (Hardwick et al., 1999). Similarly, mTOR has been identified as a direct target for the action of protein kinase B (akt) and of having a key role in insulin signalling (Shepherd et al., 1998; Navé et al., 1999). Mammalian TOR has also been implicated in the polarization of the actin cytoskeleton and the regulation of translational initiation (Alarcon et al., 1999). Phosphatidylinositol 3-kinases, such as mTOR, are functional in several aspects of the pathogenesis of tumours such as cell-cycle progression, adhesion, cell survival and angiogenesis (Roymans and Slegers, 2001).
Pharmacokinetic studies of rapamycin and rapamycin analogues have demonstrated the need for the development of novel rapamycin compounds that may be more stable in solution, more resistant to metabolic attack and/or have improved cell membrane permeability and decreased efflux and which therefore may exhibit improved oral bio-availability.
A range of synthesised rapamycin analogues using the chemically available sites of the molecule has been reported. The description of the following compounds was adapted to the numbering system of the rapamycin molecule described in FIG. 1. Chemically available sites on the molecule for derivatisation or replacement include C40 and C28 hydroxyl groups (e.g. U.S. Pat. Nos. 5,665,772; 5,362,718), C39 and C16 methoxy groups (e.g. WO 96/41807; U.S. Pat. No. 5,728,710), C32, C26 and C9 keto groups (e.g. U.S. Pat. Nos. 5,378,836; 5,138,051; 5,665,772). Hydrogenation at C17, C19 and/or C21, targeting the triene, resulted in retention of antifungal activity but relative loss of immunosuppression (e.g. U.S. Pat. Nos. 5,391,730; 5,023,262). Significant improvements in the stability of the molecule (e.g. formation of oximes at C32, C40 and/or C28, U.S. Pat. Nos. 5,563,145, 5,446,048), resistance to metabolic attack (e.g. U.S. Pat. No. 5,912,253), bioavailability (e.g. U.S. Pat. Nos. 5,221,670; 5,955,457; WO 98/04279) and the production of prodrugs (e.g. U.S. Pat. Nos. 6,015,815; 5,432,183) have been achieved through derivatisation.
However, there remains a need for a greater range of rapamycin derivatives with improved metabolic stability, improved cell membrane permeability and/or a decreased rate of efflux. Such rapamycin derivatives would have great utility in the treatment of a wide range of conditions. The present invention provides a range of 39-desmethoxyrapamycin derivatives with improved metabolic stability, improved cell membrane permeability and/or a decreased rate of efflux and/or a different cell inhibitory profile to rapamycin. Such compounds are useful in medicine, in particular for the treatment of cancer and/or B-cell malignancies, the induction or maintenance of immunosuppression, the treatment of transplantation rejection, graft vs. host disease, autoimmune disorders, diseases of inflammation, vascular disease and fibrotic diseases, the stimulation of neuronal regeneration or the treatment of fungal infections.